The present invention relates to the annealing treatment of metal articles, in particular of stainless steel, in a furnace having an entrance end zone for the metal articles to be treated connected to a heating zone followed by a cooling zone and an exit end zone for the treated metal articles. A gas having a high hydrogen content is injected in the heating zone and a dense inert gas is injected in the vicinity of said end zones. A furnace achieving this type of treatment is for example disclosed in the European patent application No. 0075,438. Usually there is injected in the annealing zone cracked ammonia having a hydrogen content of 75%, which may give rise to problems of nitriding for certain grades of steel due to the presence of nitrogen in the hot zone. A dense inert gas is injected at the ends of the furnace, which permits providing a required high pressure (usually on the order of 90 mm of water) of the gas having a high hydrogen content inside the furnace and increasing the hydrogen content of the gas injected in the active zone corresponding to the heating followed by the cooling.
It has been proposed to employ nitrogen as the dense neutral gas, but it has been found that this was not without drawbacks. Indeed, any maladjustment due to any cause, for example a drop in the internal high gas pressure of the furnace, resulting from wear of the sealing means at the entrance and the exit of the furnace or an inadaptation of the sealing means to the dimensions of a freshly introduced metal article, results in an inopportune propagation of the nitrogen toward the hot active zone which then seriously disturbs the annealing phenomenon. Such disturbance may form harmful nitriding; for example, this may occur following a sudden increase in the nitrogen admitted for compensating an occasional depression. On the other hand, if, for the purpose of compensating for a drop in the internal high pressure, the flow of hydrogen is increased one is confronted with an increase in the cost of the treatment.
This is the reason why it has been proposed to employ argon as the inert gas, whose possible presence in the hot zone is not harmful, but argon is however far more expensive than nitrogen.